DISEASE CHARACTERISTICS: Histologically or cytologically confirmed recurrent metastatic adenocarcinoma of the colon or rectum Not amenable to potentially curative treatment (e.g., inoperable metastatic disease) Must have received 2, and only 2, prior chemotherapy regimens for metastatic or recurrent disease, including the following: First-line therapy with bolus or continuous infusion fluorouracil (5-FU) (with or without leucovorin calcium) or capecitabine OR Adjuvant therapy with 5-FU and leucovorin calcium after relapse during or within 6 months of completion of first-line therapy Second-line therapy with irinotecan or after adjuvant 5-FU and leucovorin calcium treatment failure Progressive disease by CT scan or MRI during or after first-line treatment as above OR during or within 6 months of completion of adjuvant chemotherapy as above AND during second-line irinotecan or after adjuvant treatment failure as above At least 1 unidimensionally measurable lesion At least 20 mm by CT scan or MRI At least 10 mm by spiral CT scan